It is common to filter a flow of melted plastic in the high pressure line leading to an extrusion die to insure high quality of the resulting plastic products and to safeguard sensitively sized openings from plugging downstream.
Melted plastic represents a unique problem insofar that standard fluid filtering techniques are concerned by virtue of its substantial viscosity, the high temperatures at which plastic must be maintained, the narrow range of temperatures in the elevated temperature range in which plastic must be maintained to insure satisfactory flow without high temperature degradation, the extremely high pressures under which plastic is subjected to cause the flow, and the need for a homogeneous flow that not only is devoid of contaminants but of air and other gases as well.
Some of these problems are insubstantial where prime virgin plastic is used in the production run, but they are compounded for plastic when plastic trim, rejects and start-up/shut-down scrap is reground and rerun again.
Recycled plastic represents a valuable resource if it can be filtered or cleaned sufficiently before extrusion. Its quality can be greatly enhanced if it is carefully filtered to remove all of the contaminants associated with recycled materials, which include degradations of the plastic itself and all types of sizes of nonplastic particulate matter. Recycled plastic, however, creates a particularly significant problem because of the size and volume of contaminants to be removed, and the need for frequent purging of the filtering means.
Many plastic filtration devices filter efficiently, but purging requires disassembly of the unit and significant downtime while the filter screen is cleaned or replaced. In the case of recycled plastic, where the contaminants are considerable, filtering apparatus without some type of intermittently operating purge mechanism cannot effectively be used.
Other types of plastic filters include purge mechanisms, but they are usually either of insufficient capability to handle the contaminants filtered from recycled plastic, or the purge itself involves a significant waste of material during the purging process. For example, it is known to use a portion of the filtered material as a reverse flow through the filter to dislodge contaminants, but the flow of filtered plastic required for the purge is either substantial, or, if a lesser volume, insufficient to accomplish the purge.
In all cases involving the purge of filtering apparatus from melted plastic, it is essential to either avoid the entry of air or other gases into the flow or to run the plastic a period of time after the purge is completed to avoid the presence of air bubbles in the product, which makes it unacceptable.
U.S. Pat. No. 4,486,304 solves a number of these problems in filtering apparatus in which the filtering occurs continuously, but in which a purged cycle can be initiated periodically or continuously, depending on the requirements of the type of plastic used. Reference is made to U.S. Pat. No. 4,486,304 for a specific description of structure and operation of the inventive filtering apparatus. Generally speaking, it employs a conventional filter screen sandwiched between breaker plates having a substantial plurality of openings larger than the filtration openings of the screen. A collector plate having a plurality of triangularly shaped passages is disposed on the upstream side adjacent one of the breaker plates. Each one of the triangular passages creates a triangular pocket for collecting contaminants that cannot pass through the filter screen.
A rotatable collector head is disposed in engagement with the collector plate immediately upstream, including a generally triangular mouth that overlies the triangular collector pocket. The collector head is rotatable for selective registration with any of the collector pockets.
The collector head is in continuous communication with a contaminant discharge passage, and valve means are included to either block or open the discharge passage. With the discharge passage closed, it is subjected to static pressure and unable to receive any flow of plastic. With the valve open, a substantial pressure differential exists between the outlet chamber of the filter apparatus and ambient pressure, resulting in a reverse flow of filtered plastic through the filter screen, collector head and contaminant discharge passage. It is this reverse flow that releases the contaminants from the screen and carries them out for discharge.
In our prior invention, the valve means for the contaminant discharge passage is timed in cooperation with means for rotatably indexing the collector head with each of the collector pockets. With the collector head disposed in registration with a collector pocket and associated filter screen, the discharge passage is opened for a period of brief duration during which reverse purge flow is caused. The valve means then closes the discharge passage as stepped indexing of the collector head begins, and upon registration of the collector head with the next adjacent triangular pocket, the valve means is again actuated to effect reverse purge flow through this newly aligned pocket. The sequence continues until each pocket has been freed of all contaminants.
This invention includes a number of improvements over the apparatus of U.S. Pat. No. 4,486,304. Included is an improved filter assembly which in turn includes improved trap and breaker plates and segmented filter screen assemblies. The trap plate is formed with a plurality of deep, generally triangular pockets arranged in a circular array. A plurality of thin bars are integrally formed in each pocket to define passages therebetween for removing larger contaminants. These bars are disposed immediately adjacent the filter screen assembly.
The breaker plate is of similar construction, but the triangular pockets face downstream whereas the triangular pockets of the trap plate face upstream. The trap and breaker plates are mounted face-to-face with the triangular pockets in axial registration.
Generally triangular recesses are formed between the trap and breaker plates in registration with the triangular pockets to receive segmented, generally triangular filter screen assemblies. These triangular recesses may be formed in either or both the trap and breaker plates.
Each of the filter screen assemblies comprises a fine upstream filter screen and a coarser downstream filter screen. The fine screen is preferably of Dutch weave construction with a fine mesh that filters smaller contaminants on its upstream surface without permitting them to become embedded. The coarser screen provides a backing support to the fine screen during the filtering process. Equally as important, the coarser screen permits lateral flow within its mesh which not only permits a smooth outlet flow through the breaker plate as the plastic seeks its way around the thin bars of the triangular pockets, but also in the reverse purge direction evenly distributes the filtered plastic over the entire surface of the fine screen to insure that all contaminants are lifted from the fine screen during the purge process.
To insure that the reverse purge flow of filtered plastic is not excessive, an improved discharge apparatus is included that causes each pulse of contaminant-filled purge flow to enter a discharge chamber against a predetermined pressure. When a predetermined volume of contaminant-filled plastic has entered this chamber, the contaminant discharge passage is blocked off, and the slug of contaminant filled plastic is then exhausted through a discharge pipe to atmosphere.
In the preferred embodiment, a plurality of pulses of reverse flow are caused through each of the segmented filter screen assemblies to insure that all contaminants are removed.
The purging operation is cycled on a continuous basis to avoid excessive clogging of the individual filter screen assemblies. This preventive maintenance permits the filtering apparatus to operate at optimum efficiency of a continuous basis, and results in extended wear life of the filter screen assemblies. Further, this improved operation is accomplished with a minimized volume of purge flow.
Other improvements and advantages will be appreciated from the attendant specification and drawings.